The objectives of the proposed research are to elucidate biochemical mechanisms involved in rat lung growth and development. Tissue development is controlled by mutiple factors, some of which may act through cyclic AMP. In rat lungs, cyclic-AMP levels showed phasic alterations with age which corresponed to fetal, neonatal and adult stages of lung development, and were dependent on the relative activities of adenylate cyclase and cyclic AMP phosphodiestrase. On around 20th day of age, some factor(s) appeared in the lung cytosol which markedly activated adenylate cyclase and appeared to modify hormonal activation of the enzyme. Very little is known about the lung activator. However, the results of our study suggest that it is different from those described in other tissues. Studies are proposed which will examine the distribution and development of the activator in other tissues. Adenylate cyclase activator will be purified from the rat lung soluble fraction by conventional techniques and characterized in terms of its physical and chemical properties. Its role in the regulation of basal-and hormonally-stimulated adenylated cyclase will be elucidated. The nature of stimulus which caused its appearance at a particular stage of development, and whether it was newly synthesized or converted from a preexisting inactive form will be examined. The significance of this activator in the synthesis and secretion of surfactant, and in cellular multiplication and differentiation will be investigated.